In general, an exhaust heat recovery apparatus is used for heating a fluid by recovering exhaust heat of exhaust gas generated from an internal combustion engine based on the principle of heat pipe. For example, Japanese Unexamined Patent Application Publication No. 7-120178 describes a heat siphon exhaust heat recovery apparatus in which heat of exhaust gas is recovered and transferred to an engine coolant of an internal combustion engine.
The described exhaust heat recovery apparatus includes an evaporator for performing heat exchange between a heat medium and the exhaust gas and a condenser for performing heat exchange between the heat medium and the engine coolant. The evaporator and the condenser are in communication with each other through a looped passage that is filled with the heat medium. The evaporator includes U-shaped tubes, which are arranged in a flow direction of the exhaust gas. The condenser includes straight tubes that extend in a flow direction of the engine coolant.
Further, the evaporator is provided with fins on outer surfaces of the tubes to increase a heat transfer surface area, thereby to facilitate heat exchange between the heat medium and the exhaust heat. The fins are brazed with the tubes.
For example, the exhaust gas flowing around the tubes of the evaporator has temperature distribution. The temperature distribution of the exhaust gas is likely to cause a temperature difference between the tubes or between respective portions in each tube. Further, if a thermal expansion difference occurs between the tubes due to the temperature difference, the fins, which are integrated with the tubes, receive a tensile load or the like. In this case, if the fins receive an excess stress, cracks will occur at fillets of brazing between the tubes and the fins.